1. Field of the Invention
The present invention relates to a measurement apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate microdevices, for example, semiconductor devices such as an IC and LSI, an image sensing device such as a CCD, a display device such as a liquid crystal panel, and a magnetic head by using photolithography. The projection exposure apparatus transfers a pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system. The exposure apparatus is required to exactly transfer the pattern of the reticle onto the wafer at a predetermined magnification, so it is important to use a projection optical system with a good imaging performance and small aberration.
In recent years, as further micropatterning of semiconductor devices is in progress, patterns to be transferred are becoming sensitive to the aberrations of projection optical systems. This makes it necessary to accurately measure the optical performances (e.g., the wavefront aberration) of a projection optical system while it is built in an exposure apparatus.
A preferable measurement apparatus which measures the wavefront aberration of the projection optical system on the exposure apparatus is the one which attains space saving and uses a light source with low coherence. Under the circumstances, a PDI (Point Diffraction Interferometer) and a shearing interferometer have conventionally been proposed. Techniques associated with these proposals are disclosed in Japanese Patent Laid-Open Nos. 2005-159213, 2000-97666, and 2000-146705, and Daniel Malacara, “Optical Shop Testing”, John Wiley & Sons, Inc. 231 (1978).
Unfortunately, the PDI significantly decreases the light quantity of light beams which form an interference pattern, because pinholes must be formed on the object-plane side and image-plane side of an optical system to be measured (projection optical system). Particularly when a light source of an exposure apparatus is used, it is difficult to measure the wavefront aberration.
The shearing interferometer can suppress a decrease in the light quantity of light beams which form an interference pattern as compared with the PDI because pinholes need only be formed on the object-plane side of the optical system to be measured. However, the shearing interferometer outputs a measurement result (measurement value) as the derivative value of the wavefront aberration of the optical system to be measured. The absolute value of the measurement value is so small that the measurement value becomes very sensitive to a measurement error. In addition, since the measurement value must be integrated, the measurement accuracy deteriorates and the processing system is complicated. Since the entire pupil plane of the optical system to be measured cannot be measured at once, the measurement takes a long period of time.